1. Field of the Invention
This invention relates to a method of producing an information recording medium, and an information recording medium produced by the method.
2. Related Background Art
Various optical recording media and magnetic recording media are known as information recording media comprising a substrate and recording film formed thereon. An optical recording medium for recording information thereon correspondingly to a change in an optical state detectable by light permits higher density and has been put into practical use as an optical disc or an optical card. Changes in an optical state formed on the optical recording medium include recess and projection bits, the presence or absence and deformation of reflecting film, changes in refractive index, reversal of magnetization, etc. Among these, reversal of magnetization is detected as a change in the polarized state of reflected light, and another form, it is detected as a change in the quantity of reflected light.
Also, in the case of such an optical detection system, a mechanism of tracking and focusing is adopted to direct light to a predetermined place during recording and reproduction. As an element constituting the setup thereof, a groove called a guide groove is usually formed on the medium. This groove is usually not used for recording. However, in recent years, in order to make the density higher, it has been proposed to record on this groove (called the land/groove recording system) as well,, as is disclosed in Japanese Laid-Open Patent Application No. 6-34907.
In this case, the bottom surface of the groove is a flat surface parallel to a land surface and a substrate surface. The groove refers to a portion which is recessionary as viewed from a side on which information tracks are formed. In order to achieve still higher density, it is conceivable to make the track pitch small and achieve narrow tracks, and in that case, as a contrivance for not destroying the data of adjacent tracks, it has been proposed to make the level difference between a land portion and a groove portion, i.e., the depth of the groove, great, as is disclosed in Japanese Laid-Open Patent Application No. 9-161321.
In a recording mode for inducing such a change in the optical state by the application of a laser beam for heating, the size and shape of a state change area (recording mark) formed are determined by a temperature distribution induced on the recording film by localized heating. The temperature distribution induced on the recording film is affected by heat diffusion in the direction of the film surface. Therefore, even when the amount of incident heat per unit time is made constant and a heating area is moved at a constant linear velocity, the temperature distribution formed changes in a complicated manner, depending on the movement distance or the distance from a heated area.
Therefore, when a heating operation simply corresponding to information is performed, the width of a mark formed by an information pattern fluctuates. If such a mark train is time-serially read out by the scanning of a light beam, the timing jitter of mark detection increases and so, there is the risk that the original information cannot be properly reproduced.
To avoid this problem, various recording compensation systems for adjusting the amount of incident heat per unit time or the heating time or the like in conformity with the information pattern have been proposed. But, in this case, there is the problem that recording means becomes complicated.
Also, when using the known magnetron sputter method, a recording film is formed on a substrate on which the interval between information tracks is narrowed for higher density to thereby make a medium. A recording film generally uniform in the direction of the film surface is formed as shown in FIGS. 14A and 14B of the accompanying drawings. Therefore, when recording or erasing is effected on the track by the application of a laser beam for heating, there arises the problem of cross write or cross erase that information pits preserved on adjacent tracks are destroyed by the heat diffusion to the adjacent tracks through the film surface.
Also, there has been the problem that the application of a system using guide grooves for servo as well on the opposite sides of an information track as an information track (the land/groove recording system) is difficult because, as shown in FIG. 14C of the accompanying drawings, the width of the groove is narrow. The greater becomes the depth of the groove, the lesser becomes the amount of adherence of the film to the bottom of the groove, such that a recording film having a characteristic equal to a land cannot be formed on the guide groove.
Further, in Japanese Laid-Open Patent Application No. 6-290496, the inventor proposes a magneto-optical recording medium in which a first magnetic layer, a second magnetic layer and a third magnetic layer are successively laminated; the first magnetic layer has a magnetic wall coercivity smaller than that of the third magnetic layer; and the second magnetic layer has the Curie temperature lower than the Curie temperatures of the first magnetic layer and the third magnetic layer.
The inventor also proposes a high density recording-reproducing method of displacing the magnetic wall of the first magnetic layer present in the boundary portion of a recording mark by a temperature gradient by the use of this magneto-optical recording medium, and detecting the reversal of magnetization resulting from this displacement of the magnetic wall as a change in the polarized state of reflected light.
In this method, it is desirable to stabilize the displacement of the magnetic wall and to improve the reproducing characteristic that the magnetic wall of the forward boundary portion and the magnetic wall of the rearward boundary portion of the recording mark be separately formed independently of each other.
If a recording film is formed on a substrate according to the prior art to thereby make such a medium, a recording film is formed that is generally uniform in the direction of film surface. It has been difficult to form such a recording mark such that the front and rear magnetic walls are completely separate from each other. The reason why a recording film is formed that is uniform in the direction of the film surface is that molecules and atoms which have flown out of an evaporation source collide with one another before they reach the substrate or collide with inert gas atoms used during evaporation. Therefore, the directions in which they fly become random and they impinge on the substrate at various angles. Therefore, after the formation of the recording film, the opposite sides of the track have been annealed by a laser beam of high output to thereby carry out the process of causing the magnetic film on the side of the track to change in quality or disappear, and form a recording mark so as to extend over this processed portion, thereby separating the front and rear magnetic walls from each other.
There has also been the problem that, because the guide groove is annealed, the application of a system in which the guide grooves for servo on the opposite sides of an information track are also used as information tracks (the land/groove recording system) is difficult. The reason for this is that, as shown in FIGS. 13, 14B and 14C, the directions of the atoms impinging on the aforementioned substrate are random, whereby the number of atoms and molecules adhering to the land portion and adhering to the bottom of the groove before they reach the groove decreases. Therefore, the same characteristic and film thickness as those on the land are not obtained.